I attempted to find workable toasterscript code for you today. Unfortunately, it appears that Newtek killed the code as they have updated the product.
If you are using VT5, there might be a workaround for you. Open SpeedEdit and place a utility crouton on the timeline. Within the tree you can then put in the settings you desire.
My caveat: I was unable to get the MainIn Crouton to set the code you desired. I could get the line inputs to set the audio mixer as desired, but not the MainIn.
@ Newtek - can someone please inform us as to which Audio scripts work in ToasterScript? They all seem to be broken and scripting allows us to further expand the horizons.